1. Field of the Invention
The present invention relates to digital signal processing and is particularly concerned with such processing as may occur through the utilization of adaptive transversal filters.
2. Description of the Prior Art
In signal transmission in digital telecommunications systems, linear signal distortions can occur, for the equalization of which adaptive equalizers (and also compromise equalizers) are to be used, which normally consist of transversal filters. A transversal filter of this type is a delay chain which, at its input end, is fed in a stepped fashion with the signal to be processed and which, before and following each stage, (which in each case corresponds to one delay step) is provided with taps, where at each delay step the tapping signal elements which occur at each of the taps are evaluated (multiplied) in accordance with filter coefficients individually assigned to the taps, i.e. possibly amplified, attenuated and/or changed with respect to sign, and thus added to form filter output signal element. In a so-called adaptive transversal filter, the filter coefficient can be adaptively adjusted in accordance with an error signal as noted in the publications NTZ Vol. 24, No. 1, 1971, pp. 18-24 and Bocker: Datenubertragung, 1976, Vol. 1, Chapter 5.3.2 or permanently set (compromise filter). Apart from purposes of signal equalization, transversal filters can also be used in crosstalk and/or echo compensation circuits for the compensation of interference signals as set forth in the publications AGARD Conference Proceedings No. 103, 1972, pp. 12-1-12-16; Der Fernmelde-Ingenieur Vol. 31, No. 12, 1977, pp. 1-25 and the Bell System Technical Journal Vol. 58, No. 2, 1979, pp. 491-500.
In a N-1 stage transversal filter, as illustrated in FIG. 1, which is well known in the art, the value of the output signal element .sigma..sub.k, obtained in a time element k can be described by ##EQU1## wherein a.sub.k-i signifies the values of the tapped signal elements which occur in this time element at the individual taps of the delay chain, and c.sub.i signifies the N filter coefficients which determine the properties (frequency response, time characteristics) of the filter. In the case of adaptive filter adjustment, the individual filter coefficient c.sub.i can be adjusted in a stepped manner in an iteration which can be approximately described by EQU c.sub.i(k+1) =c.sub.i(k) -g.multidot..DELTA..sigma..sub.k .multidot.a.sub.k-i
wherein g represents the so-called adjustment value which determines the run-in time of the filter to the desired state, and the necessary coefficient word length, and therefore the filter accuracy, which however, in order to ensure a reliable run-in (filter convergence) should also not be selected to be too high; .DELTA..sigma..sub.k is the error, which remains in relation to a desired theoretical value, of the currently considered output signal element. Instead of such an error, it can also be possible to use only the sign sgn (.DELTA..sigma..sub.k) thereof as an adjustment criterion.
The construction of a transversal filter of this kind requires N coefficient memories, and in the processing of binary or ternary digital signals in respect of each delay step, i.e. in each time element, N tapped signal elements, evaluated in accordance with the stored filter coefficients, must be added, i.e. (a maximum of) N additions or subtractions must be carried out. In the adaptive adjustment of the filter coefficients, in respect of each delay element, N correction values must be calculated for the N coefficients. This assumes a high processing speed which corresponds to the length of the transversal filter, i.e. the value of N, which however, will not always exist for technical reasons.